Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy

Nanophotonic waveguides are at the core of a great variety of optical sensors.These structures confine light along defined paths on photonic chips and provide light-matter interaction via an evanescent field.However,waveguides still lag behind free-space optics for sensitivity-critical applications such as trace gas detection.Short optical pathlengths,low interaction strengths,and spurious etalon fringes in spectral transmission are among the main reasons why on-chip gas sensing is still in its infancy.In this work,we report on a mid-infrared integrated waveguide sensor that successfully addresses these drawbacks.This sensor operates with a 107%evanescent field confinement factor in air,which not only matches but also outperforms free-space beams in terms of the per-length optical interaction.Furthermore,negligible facet reflections result in a flat spectral background and record-low absorbance noise that can finally compete with free-space spectroscopy.The sensor performance was validated at 2.566μm,which showed a 7 ppm detection limit for acetylene with only a 2 cm long waveguide.

Ge-on-Si modulators operating at mid-infrared wavelengths up to 8 μm

We report mid-infrared Ge-on-Si waveguide-based PIN diode modulators operating at wavelengths of 3.8 and8 μm. Fabricated 1-mm-long electro-absorption devices exhibit a modulation depth of >35 dB with a 7 V forward bias at 3.8 μm, and a similar 1-mm-long Mach–Zehnder modulator has a Vπ· L of 0.47 V · cm. Driven by a 2.5 Vpp RF signal, 60 MHz on-off keying modulation was demonstrated. Electro-absorption modulation at 8 μm was demonstrated preliminarily, with the device performance limited by large contact separation and high contact resistance.